O

◊ olivine ◙ ores ◊ oxides

◊◊

alphabetic sections ◊ A ◊ B ◊ C ◊ D ◊ E ◊ F ◊ G ◊ H ◊ I ◊ J ◊ K ◊ L ◊ M ◊ N ◊ O ◊ P ◊ Q ◊ R ◊ S ◊ T ◊ U ◊ V ◊ W ◊ X ◊ Y ◊ Z ◊ elements of periodic table ◊ Mineral Index ◊ Rock Index

olivine

mineral / chemical formula		properties / significance / occurence
olivine (chrysolite, gem-quality peridot) (Mg,Fe)2SiO4.	click to enlarge - top, peridot; middle, molecular structure of olivine; bottom, close-up of peridotitie xenoliths in basalt above low power images of olivine basalt hand-specimens	Orthorhombic magnesium iron silicate with variable ratio of magnesium and iron between the two endmembers of the solid solution series: forsterite (Mg-endmember) and fayalite (Fe-endmember). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and fayalite (Fa) (e.g., Fo70Fa30). One of the commonest minerals on Earth, olivine and its polymorphs comprise over 50% of the asthenosphere. Olivine occurs in mafic and ultramafic igneous rocks and as a primary mineral in some metamorphic rocks. Mg-rich olivine crystallizes from magma rich in magnesium and low in silica, forming mafic rocks such as gabbro and basalt. Ultramafic rocks such as peridotite and dunite are more enriched in olivine after extraction of partial melts. Mg-rich olivine, or forsterite arises from metamorphism of impure dolomite or other sedimentary rocks with high Mg and low Si. Fe-rich olivine is much less common, occuring in small quantities in igneous rocks such as rare granites and rhyolites.
links: structure: olivine, olivine group (UC), (crystals UC)

images courtesy USGS

◊◊◊ Mineral Index ◊◊◊

ores

Ores are accumulations of minerals or metals in rocks that could be mined because they contain minerals that are sufficiently economically valuable, sufficiently concentrated, accessible, and extractable. Ore minerals are typically concentrations of oxides, sulfides, silicates, of metals that are not commonly concentrated in the Earth's crust, or are pure "noble" metals such as gold. Ores are mined from the ore body, then processed to extract the metals of interest from from the ore minerals. Not all mineral deposits are ores: mineral deposits are not regarded as ores when they are not economically valuable; or for minerals that are valuable, the deposit is of too low a grade or tonnage, or where extraction is technically impossible. (image : iron ore in banded iron formation, courtesy Siim Sepp)

links: images: close-up: mineral deposits: copper minerals color this vein blue-green, Phoenix gold mine, Idaho Springs

oxides

Oxides form as a result of chemical weathering (by oxygen and water) of iron and aluminum containing minerals:

Composition	Formula	Mineral name	Rock name
iron oxide	Fe2O3	Hematite	Iron ore
iron oxide	Fe3O4	Magnetite/Lodestone	Iron ore
hydrated iron oxide	2Fe2O3 & 3H2O	Limonite	Rust/iron ore
hydrated aluminum oxide	Al2O3 & 2H2O	Bauxite	Aluminum ore
aluminum oxide	Al2O3	Corundum	Ruby/sapphire